This method changes the image to black and white by changing the color mode to Grayscale. It is a destructive conversion method.
Here is a sample photograph converted using this method, and it's color original
As you can see, the resulting black and white photograph is a uniform shade of gray. Here's why:
This is a color chart with the six primary and secondary colors, along with their RGB values. For an explanation of this chart see RGB Theory.
This chart was converted using straight grayscale conversion.
This table shows the grayscale for each colored box.
The values range from 75 to 248. There isn't much difference between the six colors, especially GREEN and CYAN or MAGENTA and RED. This is the biggest drawback of using straight grayscale conversion: it turns all colors into a middling shade of gray. Although this method is simple, it also gives mediocre results.